1. Field of the Invention
This invention relates to a microelectromechanical heating apparatus and fluid preconcentrator devices utilizing same.
2. Background Art
Researchers have fabricated microheaters using thin metal as shown in references [1]-[12], poly-Si as shown in references [13]-[21], or Si as shown in references [22]-[23] on dielectric membranes with lower thermal mass for chemical sensing and other applications. The ratio of height to width of some prior art microheaters is generally smaller than 1. The range of the ratio is around 1e-4 to 1. The height/width of other microheaters varies from tens of nm/200 μm to 5 μm/5 μm. References [1]-[24] are noted in Table 1 and the list which follows the table.
TABLE 1COMPARISON OF REPORTED PRECONCENTRATOR/MICROHEATERYear/Affiliation/HeaterAdsorbentReferenceMicroheater DesignResponseMaterialAnalytesResponseComments1985, U.S. Pat.0.2 to 20 μm Pt, Rh, Pd on>700° C. atNb2O5 or CeO2O2N/AMicroheater for gas sensorNo. 4,500,412insulating substrate, such as>0.5-5 Wconsisting of[1]alumina, quartz, spinel,catalyst of Pt,magnesia, and zirconiaRh, and Pd1994, Ecole Polytech,Pt on 2 μm SiO2/Si3N4375° C.,N/ACO2, SOx,N/AMicrohotplateCanadamembrane. Serpentine design,115 mWNOx, CO,[2]0.9 × 0.9 mm2 membrane area.O2 and H2O1996, StandfordIr on SiO2/Si substrateN/AMercuryHeavy metal600/300 sLiquid phase sensorUniv., USA(Pb, Cd, Cu,for 1/10[3]etc.)ppb1997,520/50 nm Cr/Al on 520 nm90° C.,N/AN/AN/AMicroheaterPisa Univ., ItalySiO2 membrane.1.2 mW[4]Serpentine design.1997,Pt on 700/100 nm SiO2/Si3N4500° C.,SnO2Co, NO2, O3N/AThey found the heat conductionUniv. degli Studi dimembrane.130 mWthrough air is dominated butBrescia, Italy900 × 900 μm2 serpentinenot heat loss through[5]design, 1.7 × 1.7 mm2membrane or support areamembrane area.(2%).1999,5/30 nm Ti/TiN on 1 μm300° C.,N/AN/AN/AMicroheaterIMEC, BelgiumSiO2/Si substrate.138 mW[6]I line design, 1 μm wide heaterdesign.2000, Hong Kong1 μm Ba1−xLaxTiO3 on 25 nm400° C.N/AN/AN/AThin film resistor for humidityUniv., ChinaSiO2/Si substratesensor.[7]2000, Technical200 nm HfB2 on 1 μm SiC380° C.,N/AN/AN/AThe active part is separatedUniv. of Berlin,membrane.35 mWfrom the surroundingGermany80 × 80 μm2 square heater areamembrane by 6 SiC[8]on 100 × 100 μm2 membranemicrobridges.area.2001, U.S. Pat.N/A (Use conventional thinN/AH2-interactiveH2N/AMicroheater for gas sensor.No. 6,265,222film heater on the membrane).metal film (e.g.[9]Mg, Ca)covered by aH2-permeablebarrier layer(e.g. Pd, Pt)2002,Thin Pt heater on 150 μm400° C.,SnO2 (with PtExplosiveTelecommunicationO/N/O Si diaphragm.100 mWor Au asgases (e.g.Basic Research Lab,catalysts)butane,South Koreapropane, Co)[10]1994, NIST, U.S.Described in [19].500° C.,SnO2H2 and O2ResponseMicroheater for hotplate.Pat. No. 5,464,96650 mWless than[11], [13]200 s.1997, Centro480 nm n++ poly-Si on the350° C.,N/AN/AN/AMicrohotplateNacional de2000/200 nm SiO2/Si3N462 mWMicroelectron, Spainmembrane. Serpentine design,[12]0.5 × 0.5 mm2 heated area.1998, LAAS500 nm n++ poly-Si on230° C.,N/AN/ANA/MicroheaterCNRS France500/220 nm SiO2/SiN1.250 mW[14]membrane.1.6 × 1.6 mm2 microheaterarea, 3 × 3 mm2 membranearea.1998, Instituto per la450 nm n++ poly-Si on the500° C.,N/AN/AN/AMicroheater for gas sensor.Ricerca Scientifica e1150 nm SiO2 membrane.30 mWTecnologica, ItalySerpentine design, 2.5 × 2.5[15]mm2 membrane area.1999,450 nm n++ poly-Si on the400° C.,400 μm tallCo, CH4N/AMicroheater for gas phaseFerrara Univ., Italy800/200 nm SiO2/Si3N430 mWSnO2 on 0.0875detection.[16]membrane. Serpentine design.mm22000, MotorolaPoly-Si on 1.5 mm SiOxNy450° C.,SnO2N/AN/AMicrohotplateFrancemembrane.65 mW[17]2000, Univ. of0.7 μm p++ poly-Si on25 mWN/AN/AN/Ap++ Si is the structural frame.Michigan, USASiO2/Si3N4/SiO2/p++ Si.[18]Diamond grid design.1994-1996, Univ. of5 μm p++ Si underneath1200° C.,3/5 nmO2 and H2N/AMicroheater for gas sensor.Michigan, USA300/250/700 nm230 mWPt/TiO2[19], [20]SiO2/Si3N4/SiO2. Meanderdesign, 1 mm2 membrane area,0.12 mm2 sensing area.1998-2001, Sandia100/15 nm Pt/Ti on the200° C. inSurfactantDimethyl5 s for 50Gas phase preconcentrator.Lab., U.S. Pat. No.100/640 nm SiO2/Si3N411 ms,templated (ST)methylppb at a6,171,378membrane. Serpentine design,67 mWsol gelphosphonategas flow[21]-[24]5.73 mm2 membrane area.rate of3 ml/minReferences: [1] H. Takahashi et al., “Oxygen Sensor with Heater,” U.S. Pat. No. 4,500,412, 1985. [2] D. Ivanov et al., “Sputtered Silicate-Limit NASICON Thin Films for Electrochemical Sensors,” SOLID-STATE-IONICS, DIFFUSION & REACTIONS, Vol. 67, pp. 295-299, 1994. [3] R. J. Reay et al., “Microfabricated Electrochemical Analysis System for Heavy Metal Detection,” SENS. AND ACTUATORS B, Vol. 34, pp. 450-455, 1996. [4] P. Bruschi et al., “A Micromachined Hotplate on a Silicon Oxide Suspended Membrane,” PROC. OF 2ND ITALIAN CONFERENCE ON SENSORS AND MICROSYSTEMS, Rome, Italy, 1997, pp. 348-352. [5] G. Sberveglieri et al., “Silicon Hotplates for Metal Oxide Gas Sensor Elements,” MICROSYSTEM TECHNOLOGIES 3, pp. 183-190, 1997. [6] P. De Moor et al., “The Fabrication and Reliability Testing of Ti/TiN Heaters,” PROC. SPIE, Vol. 3874, PP. 284-293, 1999. [7] B. Li et al., “A New Multi-Function Thin-Film Microsensor Based on Ba1−xLaxTiO3,” SMART MATER. STRUCT., Vol. 9, pp. 498-501, 2000. [8] F. Solzbacher et al., “A Modular System of SiC-Based Microhotplates for the Application in Metal Oxide Gas Sensors,” SENS. AND ACTUATORS B, Vol. 64, pp. 95-101, 2000. [9] J. F. DiMeo et al., “Micro-Machined Thin Film Hydrogen Gas Sensor and Method of Making and Using The Same,” U.S. Pat. No. 6,265,222, 2001. [10] D.-S. Lee et al., “A Microsensor Array With Porous Tin Oxide Thin Films and Microhotplate Dangled By Wires in Air,” SENS. AND ACTUATORS B, Vol. 83, pp. 250-255, 2002. [11] M. Gaitan et al., “Micro-Holplate Devices and Methods for Their Fabrication,” U.S. Pat. No. 5,464,966, 1994. [12] A. Gotz et al., “Thermal and Mechanical Aspects for Designing Micromachined Low-Power Gas Sensors,” J. MICROMECH. MICROENG., Vol. 7, pp. 247-249, 1997. [13] T. A. Kunt et al., “Optimization of Temperature Programmed Sensing for Gas Identification Using Micro-Hotplate Sensors,” SENS. AND ACTUATORS B, Vol. 53, pp. 24-43, 1998. [14] C. Rossi et al, “Realization and Performance of Thin SiO2/SiNx Membrane for Microheater Applications,” SENS. AND ACTUATORS A, Vol. 64, pp. 241-245, 1998. [15] S. Astie et al., “Silicon Oxynitride Membrane for Chemical Sensor Application,” PROC. OF MAT. RES. SOC. SYMP, Vol. 518, pp. 99-104, 1998. [16] S. Brida et al., “Low Power Silicon Microheaters for Gas Sensors,” PROC. OF 3RD ITALIAN CONFERENCE ON SENSORS AND MICROSYSTEMS, Rome, Italy, pp. 377-382, 1999. [17] D. Vincenzi et al., “Gas-Sensing Device Implemented On A Micromachined Membrane: A Combination Of Thick-Film And Very Large Scale Integrated Technologies,” J. VAC. SCI. TECHNOL. B, Vol. 18, pp. 2441-2445, 2000. [18] C. A. Rich, “A Thermopneumatically-Actuated Silicon Microvalve and Integrated Microflow Controller,” Ph. D. Dissertation, The University of Michigan, 2000. [19] N. Najafi et al., “A Micromachined Ultra-Thin-Film Gas Detector,” IEEE TRANS. ELECTRON DEV., Vol. 41, pp. 1770-1777, 1994. [20] S. V. Patel et al., “Survivability of a Silicon-Based Microelectronic Gas Detector Structure for High-Temperature Flow Applications,” SENS. AND ACTUATORS B, Vol. 37, pp. 27-35, 1996. [21] R. P. Manginell et al. “Microfabrication of Membrane-Based Devices by HARSE and Combined HARSE/Wet Etching,” PROC. SPIE, Vol. 3511, pp. 269-276, 1998. [22] S. A. Casalnuovo et al., “Gas Phase Detection with an Integrated Chemical Analysis System,” PROC. OF THE 1999 JOINT MEETING OF THE EUROPEAN FREQUENCY AND TIME FORUM AND THE IEEE INTERNATIONAL FREQUENCY CONTROL SYMPOSIUM, Vol. 2, Besancon, France, pp. 991-996, 1999. [23] R. P. Manginell et al., “Microfabricated Planar Preconcentrator,” PROC. IEEE SOLID-STATE SENSOR AND ACTUATOR WORKSHOP, Hilton Head, SC, pp. 179-182, June 2000. [24] R. P. Manginell et al., “Chemical Preconcentrator,” U.S. Pat. No. 6,171,378, 2001. 
The analysis of complex vapor mixtures is typically performed by gas chromatography (GC) whereby a discrete sample of air is captured in a preconcentrator/focuser (PCF), introduced to the head of a polymer-coated separation column, and then eluted down the column under a positive pressure of some inert carrier gas. Separation of the components by differential partitioning along the column, which is typically ramped during the analysis to some elevated temperature, followed by detection by a downstream detector permits the determination of the mixture components by their retention times and response profiles. Traditional GC instrumentation is large and requires high power. Field portable instruments have been developed for environmental, clinical, aerospace, process control, and other applications, but remain limited by their size/weight (several kg) and power requirements (tens-to-hundreds of W).
A number of efforts have been mounted over the past 25 years to develop miniaturized GC components using Si-micromachining technology. The work of Terry et al. in 1979 was the first such effort and others have followed with varied success. “A Gas Chromatograph Air Analyzer Fabricated on a Silicon Wafer”, IEEE Trans. Electron Dev., vol. 26, pp. 1880-1884, 1979. The system reported recently by Frye-Mason et al. at Sandia National Laboratories, developed primarily for detection of chemical warfare agents, combines an adsorbent-coated, heated-membrane preconcentrator with a 1-m etched-Si separation column and a detector consisting of an integrated array of three surface acoustic wave sensors, and represents the most comprehensive effort, to date, to construct an entirely microfabricated system. “Hand-Held Miniature Chemical Analysis System (μChemLab) for Detection of Trace Concentrations of Gas Phase Analytes”, in Proc. of Micro Total Analysis Systems (μ-TAS) '00 Workshop, Enschede, Netherlands, pp. 229-232, May 2000.
There is a need for a more sophisticated monolithic microscale GC (μGC) for the analysis of complex vapor mixtures encountered in the ambient, indoor environment, breath, chemical processing equipment, and head-space samples of soil or other materials contaminated with organic compounds that give rise to vapor contamination in the air at concentrations as low as parts-per-billion (ppb), as shown in FIG. 1. The key components of such a μGC are shown in FIG. 2. An inlet filter 10 prevents particle entrainment and an on-board vapor generator provides an internal standard for calibration, quality control, system diagnostics, and temperature compensation. A multi-stage adsorbent PCF 14 collects vapors spanning a wide range of vapor pressures with adequate capacity to achieve detection limits in the low-ppb concentration range while also producing narrowly focused injection plugs upon thermal desorption (with reversal of flow direction) for efficient high-speed separations. A dual-column separation stage 16 allows the retention of components to be adjusted via temperature programming and/or pressure programming to maximize resolution and minimize analysis time. Detection by a sensor array 18 yields a fingerprint of eluting analytes, much like a mass spectrometer, which will aid in identifying unknowns from mixtures of arbitrary composition. Various microvalves 20 including a tuning valve 21 direct sample flow through the system under the suction pressure provided by a system diaphragm micropump 22. An internal standard 23 is also provided.
Sample collection and injection onto the column are important factors. A sufficient sample volume (or mass) is required so that quantitative analysis of each vapor component is possible at desired detection limits, and the column injection volume must be small in order to minimize dilution, referred to as inlet band broadening, which reduces the resolving power of the column. Thus, the PCF 14 must contain sufficient adsorbent mass (surface area) to ensure quantitative trapping of vapors from the sample stream, but small enough to be rapidly heated to ensure complete desorption and to minimize the desorbed-vapor bandwidth. Minimizing the power required for heating is also important.
Conventional preconcentrators, or so-called microtraps, consist of a stainless-steel or glass capillary tube packed with one or more granular adsorbent material. For desorption, a current is passed through the stainless-steel tube or through a metal wire coiled around the glass capillary tube. Capillary tubes suffer from large dead volume and limited heating efficiency due to their larger thermal mass.
Micromachining technology can overcome these limitations by significantly reducing the dead volume and thermal mass. Microheaters fabricated on dielectric membranes with low thermal mass have been reported for chemical sensing and other applications. Similar structures coated with thin adsorbent films are used for preconcentration and focusing in the Sandia microsystem referred to in reference [24]. Although rapid thermal desorption at relatively low power can be achieved with such structures, the capacity of the PCF is very low and therefore not suitable for quantitative analysis of multi-vapor mixtures. As the adsorbent layer thickness is increased to reach sufficient capacity, the thermal transfer efficiency from the thin heater on the membrane decreases dramatically, calling for alternative heater designs.